Integrated circuits are typically housed within a package that is mounted to a printed circuit board (PCB). The package is designed to protect the integrated circuit device from damage, to provide adequate heat dissipation during operation, and to provide electrical connection between the integrated circuit device and a PCB (e.g., a peripheral card, a motherboard and the like). These conventional packages may include a variety of packages such as pin grid array (PGA), land grid army (LGA), ball grid array (BGA), column grid array (CGA), and other packages. The present invention is directed at improving the reliability of the electrical/mechanical connection formed between a package and a PCB.
A BGA package is an integrated circuit package which has a plurality of solder balls that interconnect the package to a PCB. A BGA package typically consist of a substrate having an array of electrical contact pads disposed about the substrate's bottom surface. Using existing techniques, spherical shaped solder balls are attached to the package by first applying a resin flux to the electrical contact pads, positioning the solder balls onto the electrical contact pads, and running the package through a reflow furnace. FIG. 1 illustrates the solder ball, flux and electrical contact pad configuration before the reflow process. During the reflow process the solder balls are held in position by the flux and wetted onto the electrical contact pads. In addition to holding the solder balls in position, the flux promotes the wetting of the solder balls to the contact pads and chemically cleans the contact pad surfaces. As shown in FIG. 3, after having passed through the reflow furnace, the solder balls have a truncated portion located near the solder ball and electrical contact pad interface, but in all other respects maintain their spherical shape. In lieu of using resin flux, other methods utilize supporting fixtures to hold the solder balls in position during reflow with or without the use of flux.
One problem associated with using solder balls is that, because of their spherical shape, they tend to roll out of position during the reflow process. As a result, the current methods require either that a fixture be used or that additional resin flux be applied to the contact pads to ensure that the solder balls remain in position during reflow. There are, however, a number of problem associated with the current methods of forming solder balls on the contact pads of BGA packages. To begin with, the use of additional flux to hold the solder balls in position is undesirable since it results in the need to remove the surplus flux after reflow. The use of a fixture to hold the solder balls in position is also undesirable in that it involves an additional manufacturing step and requires additional manufacturing hardware to bond the solder ball to the package during reflow. In addition, the solder balls themselves are costly items due to the elaborate method of manufacture required to ensure solder ball diameter uniformity.
As previously discussed, other methods are also used to form the electrical and mechanical connection between a package and a printed circuit board. One such method includes the use of column grid arrays. Column grid arrays are used in applications where the alignment between the electrical contact pads of a package and a PCB significantly vary during the manufacturing process causing high shear stresses to occur at the electrical contact pad and solder interfaces. In particular, column grid arrays are most often used in applications when large packages are involved and where the coefficient of thermal expansion of a package greatly differs from the coefficient of thermal expansion of the PCB.
Column grid arrays include a plurality of solid solder columns that are positioned between the electrical contact pads of a package and a PCB. The connection between the electrical contact pads on a package and PCB are generally formed by first applying a resin flux to the electrical contact pads, positioning the solder columns between the pads, and running the unit through a reflow furnace. The solder column height, which is greater than the diameter of a typical solder ball, allows for greater flexibility in establishing the connection between a package and a PCB. By allowing for greater flexibility, the solder columns reduce stresses at the solder column/electrical contact pad interfaces which, in turn, results in higher product reliability.
One problem associated with using solder columns lies in their susceptibility to collapse and form solder balls during reflow which often results in bad electrical and mechanical connections.
What is needed then is a method which solves the aforementioned problems associated with forming reliable connections between a package and a PCB. As will be seen, the present invention provides an improved apparatus and method for forming column grid arrays and an improved method for manufacturing packages having solder balls which solve the aforementioned problems.